If all three are collected, a knife familiar is assembled which charges across the room and damages enemies.
The knife familiar also grants access to the Corpse.
Some familiars will follow the knife familiar across the room instead of Isaac.
How To Acquire
Knife Piece 1 is located in Downpour II. Somewhere on the floor, there will be a seemingly empty dead-end room with a minimap icon that indicates the room has a chest. Bombing the center of the room will open a hidden stone chest containing Knife Piece 1.
Knife Piece 2 is located in Mines II. Somewhere on the floor, there will be a room containing four buttons. Next to each button is 1 to 4 small pebbles, indicating the order in which to press them. After pressing the four buttons, Knife Piece 2 will appear in the center of the room.
Knife Piece 3 is located in Mausoleum II. Somewhere on the floor, there will be a room containing many fireplaces. Somewhere else on the floor, there will be a room containing active and inactive teleporters in the same formation as the fireplaces. To acquire Knife Piece 3, only put out the fires in the fireplace room that correspond with the inactive teleporters and the third Knife Piece will appear.
Try to not take Abel or Lil' Dumpy as these familiars might put out fires that should stay active.
After acquiring a Knife Piece, its puzzle will not need to be solved again in future runs, and the piece will automatically appear in the center of the room.